This invention relates to a master cylinder for use in a hydraulic braking system of a vehicle such as an automobile and, particularly to a tandem type master cylinder incorporating a hydraulic pressure control valve.
A tandem type master cylinder incorporating a hydraulic pressure control valve has been proposed and widely utilized in a hydraulic braking system of a vehicle. The tandem master cylinder supplies pressurized fluid to two independent braking circuits such as a front wheel circuit and a rear wheel circuit. The pressure control valve controls the pressure in the rear wheel circuit as compared with the front wheel circuit. U.S. Pat. Nos. 3,970,347 and 4,477,122, Japanese Patent Publication No. 58-3851 and Japanese Patent Public Disclosure No. 60-135355 are typical examples of such prior art arrangements.
Typically, the pressure control valve or pressure proportioning valve includes a differential pressure piston working in a bore and having different pressure receiving areas on opposite ends thereof, an axial passage provided in the piston and a valve member cooperating with one end of the passage. A chamber facing a small end or an end having a small effective pressure receiving area (inlet chamber) is connected to the outlet port of the master cylinder, and a chamber facing a large end or an end having a large effective pressure receiving area (outlet chamber) is connected to the rear wheel circuit. When the pressure supplied to the valve mechanism exceeds a predetermined level, the piston moves toward the small end and the valve member cuts off the communication between the chambers (a cut-off pressure). When the pressure in the inlet chamber further increases, the piston moves toward the outlet chamber to permit a small amount of liquid flow from the inlet chamber to the outlet chamber and returns to valve closing position, whereby the pressure in the outlet chamber increases at a reduced rate as compared with the pressure in the inlet chamber.
U.S. Pat. No. 3,970,347 discloses a brake pressure control valve for use with a dual type brake master cylinder and connected to two independent pressure circuits. The valve comprises an elongated housing having an axial bore therein, and two pressure proportioning valve mechanisms are provided in the opposite end portions of the bore in mutually aligned relationship. The valve further includes a failure warning mechanism including a balance piston arranged between the two differential pistons of the pressure proportioning mechanisms and being displaceable to actuate an electric switch when a failure such as a fluid leakage exists in one of the pressure circuits.
However, these prior art devices have a shortcoming such that even when there exists a failure such as a liquid leakage in the front circuit the pressure proportioning valve in the rear wheel circuit controls the brake pressure applied to rear wheels similarly to the normal operating condition, whereby the braking force acting on the vehicle is reduced about one fourth and, accordingly, it is required to apply an excessibly large braking force on a brake pedal for safely stopping the vehicle.
For preventing the aforesaid shortcoming, an actuating member has been provided to deactivate or open the control valve in response to failure in either one of the braking circuits. According to prior art devices, the actuating member comprises a tiltable element or a transversely displaceable element and one end of the element projects transversely through a circumferential wall of a cylinder for engaging with a piston or a plunger received in the cylinder. When the piston or the plunger is a member separate from the master cylinder such as shown in U.S. Pat. No. 4,447,122, the size of the master cylinder must be increased and the hydraulic system is complicated. The tiltable element according to the prior art requires a relatively large amount of displacement for the piston or the plunger for reducing the torque acting on the pivot of the tiltable element and improving the strength or rigidity of the tilting element.